The generation of heat causes degradation of characteristics in pyrogenic parts of electronic instruments, such as power transistors and thyristors, and integrated circuit elements, such as IC and LSI. Therefore, it is necessary to provide some means within the electronic instruments to smoothly radiate heat.
The advancements in high density integration of electronic instruments, such as computers and word processors, have increased the amount of heat generated in pyrogenic parts, such as power transistors and thyristors, and in integrated circuit elements, such as IC and LSI, within these instruments. Therefore, a conventional forced air cooling system utilizing cooling fans no longer provides sufficient cooling. Particularly, in the case of portable personal computers of the lap-top type and notebook type, a cooling method besides the forced air cooling type becomes necessary. Further, these particular electronic instruments have the integrated circuit elements attached to a printed substrate board, the materials for which are often glass reinforced epoxy resins and polyimide resins with inferior thermal conductivity. As a result, these instruments cannot release the heat from the substrate board through a radiative insulating sheet, as commonly performed in conventional models. Therefore, the conventional heat radiation method employs radiators of a spontaneous cooling type or a forced cooling type which are placed in the vicinity of the integrated circuit elements and the heat generated from the elements is conducted to and is radiated from these radiators. In this method, a radiative insulting sheet is placed between the pyrogenic elements and the radiators for better heat conductivity. However, the conventional radiative insulating sheets are inferior in their flexibility, which may cause the problem of damage to the adjacent elements and to the substrate board due to the excess stress developed during thermal expansion. Therefore, a radiative insulating sheet with improved flexibility is in demand.
Further, when the heat radiation is carried out by the attached radiators, attachment of radiators for each integrated circuit will require extra space, which makes building the compact instrument difficult. Therefore, the current type combines several pyrogenic elements with one radiator. In this case, each element has a different height, thus the sizes and the shapes of gaps between the elements and the radiators are widely varied. A radiative insulating sheet which corresponds to this problem is also in demand. Some thermally conductive materials which offer a superior thermal conductivity, a good flexibility, and a capability to correspond to a variety in shapes and sizes of gap, have been proposed.
Japanese Patent Application Kokai (laid open) 2-166755 discloses a method which employs a sheet formed from a mixture of silicone resin gels and thermally conductive materials such a metal oxides, wherein grooves are placed on one side or both sides of the sheet. Such grooves are intended to increase the variability in amount, leading to an increase in the contact surface area. However, this sheet is so soft that its handling is difficult after forming, and mass production is also difficult. Further, there are also disadvantages such as insufficient strength and a time-consuming assembly process.
Japanese Patent Application Kokai 2-196453 discloses a sheet which comprises a silicone resin layer formed from mixture of silicone resins and thermally conductive materials, such as metal oxides, having enough strength for handling, and a soft flexible silicone resin layer, being laid on the resin layer. This sheet is easier in handling, however, its forming method is difficult and it is not suitable for mass production. The silicone resin layer possesses a certain strength and when it is formed together with a softer silicone resin layer, it will be deformed by the stress occurring during the forming process. As a result, this sheet is not suitable for the common forming methods, such as automatic injection forming and continuous coating forming processes, which results in low productivity and a high cost.
A radiative insulating sheet reinforced by cloth-type reinforcing materials, such as a glass cloth, is also known (Japanese Patent Application Kokai 56-161140). This sheet is produced by laminating two silicone rubber sheets, containing silane-treated inorganic fillers, on both sides of a silane-treated glass cloth. Therefore, its productivity is low and the two thermally conductive silicone rubber sheets at the top and the bottom are not in contact, which impairs the thermally conductivity.